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In a scene straight out of CSI, jurors in the Amanda Knox murder trial heard about invisible—and damning—bloody footprints that reappeared after some forensic magic.

There are shiny new ventilation pipes on the “house of horrors” in central Perugia, Italy, where British student Meredith Kercher was sexually assaulted, tortured, and strangled before dying from a knife wound to her neck in November 2007. The notorious crime scene is being tidied up. Pink roses bloom in the garden. A bouquet of dead flowers with a faded card “for Mez” hangs from the gate. Soon the broken window will be fixed, and the blood in Kercher’s room chemically blasted from the floor and walls. The owner hopes to rent it out to students in the fall.

While the owners of the little house are getting on with their lives, the suspects accused of killing Kercher are not. In what may be a pivotal CSI moment in the courtroom in Perugia on Saturday, the jury heard damning evidence against both American student Amanda Knox and her Italian former boyfriend, Rafaelle Sollecito, who are being tried for the murder. Lorenzo Rinaldi, director of the print-identity division of Italy’s forensic police, told the court that investigators had used a chemical called Luminol to expose a footprint attributed to Knox outside Kercher’s bedroom, pointed toward her door. “Luminol reacts to rust, bleach, and various types of fruit juice,” he testified. “But in forensics we generally use it to find traces of blood”—even if someone has tried to clean that blood away.

On Saturday, the jury also heard that a bloody footprint found on a bathroom rug near Kercher’s bedroom did not belong to Rudy Guede, the man who has already been convicted for his part in Kercher’s sexual assault and murder. Projecting images onto a giant screen on the brick wall of the courtroom, Rinaldi showed the court various comparisons of footprints taken from Knox, Sollecito, and Guede to those left in blood at the crime scene. He superimposed images of Guede’s and Sollecito’s footprints over the bloody print on the rug. “You can see clearly that this bloody footprint on the rug does not belong to Mr. Guede,” he said. “But you can see that it is compatible with Sollecito.”

Finding Sollecito’s naked footprint in the bathroom of his girlfriend’s house is hardly damning. But the footprint on the rug is clearly in blood that has been identified as Kercher’s. Sollecito does not deny being in the house after the murder, and in fact shoe footprints compatible with his size 9 Airforce 1 Nike trainers have also been identified throughout the house. But he has never admitted to being barefoot that morning. Likewise, finding Knox’s footprints in her own house prove nothing—except that these prints were only discovered using Luminol, which suggests, according to the prosecution, that the original prints might have been cleaned up.

Worse for Knox, when the judge asked Rinaldi the size of an unidentified bloody shoeprint found on the pillow below Kercher’s body, he responded, “Between 36 and 38.” The judge then asked Rinaldi what size shoe Knox wears. “The Skecher shoe we sequestered belonging to Amanda Knox corresponds with size 37.”

Curt Knox, Amanda's father, who is now returning to Seattle, spoke briefly with reporters outside the courtroom: "I think it could have been more favorable," he said. "I think when our experts come in and Sollecito's experts come in, you are going to get a different position." Either because he doesn’t understand Italian, or simply because he’s in denial, he didn’t seem to grasp the full impact of the day’s testimony. When a reporter asked whether he was surprised to hear about his daughter’s bloody footprints in the house, he said, ''I heard ‘compatible with size and shape.’ It wasn't a definite ID as with fingerprints, it could have been my feet or your feet.''

Saturday in court was gripping, but Friday was surreal due in part to a series of technical glitches in the prosecution. At one point, Sollecito, who recently finished his I.T. degree in prison, stood at the prosecutor’s table to troubleshoot the computer, because the video CD of the crime scene wouldn’t play. And since it may well have contained evidence that would incriminate him, one couldn’t help wondering whether Sollecito might be tempted to wipe the hard drive clean, or at least start deleting files.

“When the judge asked Rinaldi the size of an unidentified bloody shoeprint found on the pillow below Kercher’s body, he responded, “Between 36 and 38.” The judge then asked Rinaldi what size shoe Knox wears. “The Skecher shoe we sequestered belonging to Amanda Knox corresponds with size 37.”

Friday’s hearing had also just begun when news broke of another grisly murder involving a young American studying in Italy. Twenty-four-year-old Jonathan Robert Hindenach of Olivet, Michigan, is accused of murdering a 62-year-old pensioner with a shard of glass in Florence. Hindenach, who reportedly suffered from mental problems, followed the man into an alley and stabbed him to death for no apparent reason. Then he reportedly took off his clothes and ran around in his bloodied boxer shorts screaming “I killed him.” Suddenly and sadly, the bizarre circumstances of the Kercher murder seemed somehow less anomalous.

Throughout the weekend, American “trial tourists” sauntered in and out of the courtroom to see Knox. One couple told The Daily Beast that because they’d heard so much about the case, they couldn’t resist seeing Knox in person. They had also gone to see the house where Kercher was killed.

The Daily Beast also discovered that administrators of one of the blogs that follows the Kercher trial, True Justice for Meredith Kercher, is considering buying the House of Horrors to make sure it isn't forgotten. “Increasingly, Meredith's followers seem to hope that the groundswell for Meredith evolves into something tangible. Making an offer for the Via della Pergola house, perhaps establishing a memorial garden there, is one possible objective,” Peter Quennell, who runs the True Justice site, tells me. “Meredith is clearly coming to stand for something transcendent. She already seems an iconic presence for many followers of the case.”

Barbie Nadeau has reported from Italy for Newsweek magazine since 1997. She also writes for CNN Traveller, Budget Travel Magazine and Frommer's.